See if you can use anything from here. I also had a discussion on this some time back about coaxial pipes.

I did some thinking on this matter some time ago and unfortunately this is project is doomed to fail unless you add in an extra tweeter.

The problem is the TOTAL amount of treble generated by the speaker is significantly less than the mids and the bass. The treble is only on axis. When you disperse it around, the amount of treble will reduce significantly as the off axis has no treble component.

May I suggest you have a look at the linkwitz LX mini for inspiration? Somewhat similar in concept. Instead of sealed, use it as a coaxial transmission line.

An alternative could be to use a coaxial with a compression tweeter like a eminence beta. The tweeter is 10 db more than the woofer.

By the way. Don't give up on the coaxial transmission line. Cause I want to build it some day too and your experience could be useful..

Some thoughts. If you wanted front facing speaker add in a T junction. Alternatively cut a square hole at the side of the tube and use wood or something to make a Baffle. Get wooden vinyl to wrap your tubes.